[PATCH] splice: fix problem with sys_tee and SPLICE_F_NONBLOCK

[PATCH] splice: fix problem with sys_tee and SPLICE_F_NONBLOCK

[Johann Felix Soden]

From: Johann Felix Soden <johfel@users.sourceforge.net>

With SPLICE_F_NONBLOCK sys_tee should return number of duplicated bytes,
not only -EAGAIN on success.

This patch also solves the problem, which is described on
http://article.gmane.org/gmane.linux.kernel/642502.

Signed-off-by: Johann Felix Soden <johfel@users.sourceforge.net>
---
 fs/splice.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/fs/splice.c b/fs/splice.c
index 9b559ee..184fd66 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -1711,7 +1711,7 @@ static long do_tee(struct file *in, struct file *out, size_t len,
 			ret = link_opipe_prep(opipe, flags);
 			if (!ret) {
 				ret = link_pipe(ipipe, opipe, len, flags);
-				if (!ret && (flags & SPLICE_F_NONBLOCK))
+				if (ret < 0 && (flags & SPLICE_F_NONBLOCK))
 					ret = -EAGAIN;
 			}
 		}
-- 
1.5.4.2

Re: [PATCH] splice: fix problem with sys_tee and SPLICE_F_NONBLOCK

[Jens Axboe]

On Tue, Feb 19 2008, Johann Felix Soden wrote:
> From: Johann Felix Soden <johfel@users.sourceforge.net>
> 
> With SPLICE_F_NONBLOCK sys_tee should return number of duplicated bytes,
> not only -EAGAIN on success.

?

The current behaviour is to return bytes tee'd, or return -EAGAIN for
zero bytes if SPLICE_F_NONBLOCK is set. It doesn't return "-EAGAIN on
success", not sure what you mean there.

> This patch also solves the problem, which is described on
> http://article.gmane.org/gmane.linux.kernel/642502.
> 
> Signed-off-by: Johann Felix Soden <johfel@users.sourceforge.net>
> ---
>  fs/splice.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/splice.c b/fs/splice.c
> index 9b559ee..184fd66 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -1711,7 +1711,7 @@ static long do_tee(struct file *in, struct file *out, size_t len,
>  			ret = link_opipe_prep(opipe, flags);
>  			if (!ret) {
>  				ret = link_pipe(ipipe, opipe, len, flags);
> -				if (!ret && (flags & SPLICE_F_NONBLOCK))
> +				if (ret < 0 && (flags & SPLICE_F_NONBLOCK))
>  					ret = -EAGAIN;
>  			}
>  		}

Perhaps it's just me, but this doesn't make a lot of sense. You override
any other error with EAGAIN, hm? In fact the only < 0 value that
link_pipe() will return is -EPIPE, which is perfectly in sync with that
a pipe write will return if there are no readers attached.

-- 
Jens Axboe

Re: [PATCH] splice: fix problem with sys_tee and SPLICE_F_NONBLOCK

[Johann Felix Soden]

> On Tue, Feb 19 2008, Johann Felix Soden wrote:
> > From: Johann Felix Soden <johfel@users.sourceforge.net>
> > 
> > With SPLICE_F_NONBLOCK sys_tee should return number of duplicated bytes,
> > not only -EAGAIN on success.
> 
> ?
> 
> The current behaviour is to return bytes tee'd, or return -EAGAIN for
> zero bytes if SPLICE_F_NONBLOCK is set. It doesn't return "-EAGAIN on
> success", not sure what you mean there.
> 
Sorry, my patch description was not correct. 

The new behavior of sys_tee with my patch is: 
	- return -EAGAIN if there are no data in the pipe, but writer
	  connected to the pipe, 
	- return 0 if there are not writers connected
	- else return number of duplicated byte 

The old behavior was: return -EAGAIN or the number (>0) of duplicated
bytes.

> > This patch also solves the problem, which is described on
> > http://article.gmane.org/gmane.linux.kernel/642502.
> > 
> > Signed-off-by: Johann Felix Soden <johfel@users.sourceforge.net>
> > ---
> >  fs/splice.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/fs/splice.c b/fs/splice.c
> > index 9b559ee..184fd66 100644
> > --- a/fs/splice.c
> > +++ b/fs/splice.c
> > @@ -1711,7 +1711,7 @@ static long do_tee(struct file *in, struct file *out, size_t len,
> >  			ret = link_opipe_prep(opipe, flags);
> >  			if (!ret) {
> >  				ret = link_pipe(ipipe, opipe, len, flags);
> > -				if (!ret && (flags & SPLICE_F_NONBLOCK))
> > +				if (ret < 0 && (flags & SPLICE_F_NONBLOCK))
> >  					ret = -EAGAIN;
> >  			}
> >  		}
> 
> Perhaps it's just me, but this doesn't make a lot of sense. You override
> any other error with EAGAIN, hm? In fact the only < 0 value that
> link_pipe() will return is -EPIPE, which is perfectly in sync with that
> a pipe write will return if there are no readers attached.

The reason, why I wrote the patch, was, that the example program
mentioned in manpage of tee(2) doesn't stop, if it used like
cat textfile | tee_example outfile | wc because sys_tee returns only
-EAGAIN or the duplicated bytes, not something like EOF.

See also http://article.gmane.org/gmane.linux.kernel/642502. 
Patrick McManus described the problem there.

--

Johann Felix Soden

Re: [PATCH] splice: fix problem with sys_tee and SPLICE_F_NONBLOCK

[Jens Axboe]

On Tue, Feb 19 2008, Johann Felix Soden wrote:
> > On Tue, Feb 19 2008, Johann Felix Soden wrote:
> > > From: Johann Felix Soden <johfel@users.sourceforge.net>
> > > 
> > > With SPLICE_F_NONBLOCK sys_tee should return number of duplicated bytes,
> > > not only -EAGAIN on success.
> > 
> > ?
> > 
> > The current behaviour is to return bytes tee'd, or return -EAGAIN for
> > zero bytes if SPLICE_F_NONBLOCK is set. It doesn't return "-EAGAIN on
> > success", not sure what you mean there.
> > 
> Sorry, my patch description was not correct. 
> 
> The new behavior of sys_tee with my patch is: 
> 	- return -EAGAIN if there are no data in the pipe, but writer
> 	  connected to the pipe, 
> 	- return 0 if there are not writers connected
> 	- else return number of duplicated byte 
> 
> The old behavior was: return -EAGAIN or the number (>0) of duplicated
> bytes.

Your patch has an odd way of achieving that goal, modify the real
location of the assignment instead of overriding something. That has the
potential to turn into another confusing bug later on, wondering why the
heck your return value isn't being passed back.

Improvement is welcome though, you can't distuingish -EAGAIN on the
input side from the output side currently.

-- 
Jens Axboe

Re: [PATCH] splice: fix problem with sys_tee and SPLICE_F_NONBLOCK

[Johann Felix Soden]

Am Dienstag, den 19.02.2008, 22:25 +0100 schrieb Jens Axboe:
> On Tue, Feb 19 2008, Johann Felix Soden wrote:
> > > On Tue, Feb 19 2008, Johann Felix Soden wrote:
> > > > From: Johann Felix Soden <johfel@users.sourceforge.net>
> > > > 
> > > > With SPLICE_F_NONBLOCK sys_tee should return number of duplicated bytes,
> > > > not only -EAGAIN on success.
> > > 
> > > ?
> > > 
> > > The current behaviour is to return bytes tee'd, or return -EAGAIN for
> > > zero bytes if SPLICE_F_NONBLOCK is set. It doesn't return "-EAGAIN on
> > > success", not sure what you mean there.
> > > 
> > Sorry, my patch description was not correct. 
> > 
> > The new behavior of sys_tee with my patch is: 
> > 	- return -EAGAIN if there are no data in the pipe, but writer
> > 	  connected to the pipe, 
> > 	- return 0 if there are not writers connected
> > 	- else return number of duplicated byte 
> > 
> > The old behavior was: return -EAGAIN or the number (>0) of duplicated
> > bytes.
> 
> Your patch has an odd way of achieving that goal, modify the real
> location of the assignment instead of overriding something. That has the
> potential to turn into another confusing bug later on, wondering why the
> heck your return value isn't being passed back.
> 
> Improvement is welcome though, you can't distuingish -EAGAIN on the
> input side from the output side currently.
> 

I thought again about the problem and my patch: you are right, the patch
is nonsense. I have learnt, that the correctness of a patch is not
guaranteed by the (bad, but anyhow working) solution of the problem the
patch was written for.
Sorry for wasting your time.
--
 J. F. Soden

Re: [PATCH] splice: fix problem with sys_tee and SPLICE_F_NONBLOCK

[Jens Axboe]

On Tue, Feb 19 2008, Johann Felix Soden wrote:
> 
> Am Dienstag, den 19.02.2008, 22:25 +0100 schrieb Jens Axboe:
> > On Tue, Feb 19 2008, Johann Felix Soden wrote:
> > > > On Tue, Feb 19 2008, Johann Felix Soden wrote:
> > > > > From: Johann Felix Soden <johfel@users.sourceforge.net>
> > > > > 
> > > > > With SPLICE_F_NONBLOCK sys_tee should return number of duplicated bytes,
> > > > > not only -EAGAIN on success.
> > > > 
> > > > ?
> > > > 
> > > > The current behaviour is to return bytes tee'd, or return -EAGAIN for
> > > > zero bytes if SPLICE_F_NONBLOCK is set. It doesn't return "-EAGAIN on
> > > > success", not sure what you mean there.
> > > > 
> > > Sorry, my patch description was not correct. 
> > > 
> > > The new behavior of sys_tee with my patch is: 
> > > 	- return -EAGAIN if there are no data in the pipe, but writer
> > > 	  connected to the pipe, 
> > > 	- return 0 if there are not writers connected
> > > 	- else return number of duplicated byte 
> > > 
> > > The old behavior was: return -EAGAIN or the number (>0) of duplicated
> > > bytes.
> > 
> > Your patch has an odd way of achieving that goal, modify the real
> > location of the assignment instead of overriding something. That has the
> > potential to turn into another confusing bug later on, wondering why the
> > heck your return value isn't being passed back.
> > 
> > Improvement is welcome though, you can't distuingish -EAGAIN on the
> > input side from the output side currently.
> > 
> 
> I thought again about the problem and my patch: you are right, the patch
> is nonsense. I have learnt, that the correctness of a patch is not
> guaranteed by the (bad, but anyhow working) solution of the problem the
> patch was written for.
> Sorry for wasting your time.

Don't worry, it's not a waste of time even though your solution isn't
the correct one.

When non-blocking is set, ideally we want to return 0 if there's no hope
of anymore data and EAGAIN if trying later may yield some data. So how
about this instead?

diff --git a/fs/splice.c b/fs/splice.c
index 9b559ee..0670c91 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -1669,6 +1669,13 @@ static int link_pipe(struct pipe_inode_info *ipipe,
 		i++;
 	} while (len);
 
+	/*
+	 * return EAGAIN if we have the potential of some data in the
+	 * future, otherwise just return 0
+	 */
+	if (!ret && ipipe->waiting_writers && (flags & SPLICE_F_NONBLOCK))
+		ret = -EAGAIN;
+
 	inode_double_unlock(ipipe->inode, opipe->inode);
 
 	/*
@@ -1709,11 +1716,8 @@ static long do_tee(struct file *in, struct file *out, size_t len,
 		ret = link_ipipe_prep(ipipe, flags);
 		if (!ret) {
 			ret = link_opipe_prep(opipe, flags);
-			if (!ret) {
+			if (!ret)
 				ret = link_pipe(ipipe, opipe, len, flags);
-				if (!ret && (flags & SPLICE_F_NONBLOCK))
-					ret = -EAGAIN;
-			}
 		}
 	}
 

-- 
Jens Axboe

Re: [PATCH] splice: fix problem with sys_tee and SPLICE_F_NONBLOCK

[Johann Felix v. Soden-Fr.]

On Wed, Feb 20, 2008 at 10:35:28AM +0100, Jens Axboe wrote:
> On Tue, Feb 19 2008, Johann Felix Soden wrote:
> > 
> > Am Dienstag, den 19.02.2008, 22:25 +0100 schrieb Jens Axboe:
> > > On Tue, Feb 19 2008, Johann Felix Soden wrote:
> > > > > On Tue, Feb 19 2008, Johann Felix Soden wrote:
> > > > > > From: Johann Felix Soden <johfel@users.sourceforge.net>
> > > > > > 
> > > > > > With SPLICE_F_NONBLOCK sys_tee should return number of duplicated bytes,
> > > > > > not only -EAGAIN on success.
> > > > > 
> > > > > ?
> > > > > 
> > > > > The current behaviour is to return bytes tee'd, or return -EAGAIN for
> > > > > zero bytes if SPLICE_F_NONBLOCK is set. It doesn't return "-EAGAIN on
> > > > > success", not sure what you mean there.
> > > > > 
> > > > Sorry, my patch description was not correct. 
> > > > 
> > > > The new behavior of sys_tee with my patch is: 
> > > > 	- return -EAGAIN if there are no data in the pipe, but writer
> > > > 	  connected to the pipe, 
> > > > 	- return 0 if there are not writers connected
> > > > 	- else return number of duplicated byte 
> > > > 
> > > > The old behavior was: return -EAGAIN or the number (>0) of duplicated
> > > > bytes.
> > > 
> > > Your patch has an odd way of achieving that goal, modify the real
> > > location of the assignment instead of overriding something. That has the
> > > potential to turn into another confusing bug later on, wondering why the
> > > heck your return value isn't being passed back.
> > > 
> > > Improvement is welcome though, you can't distuingish -EAGAIN on the
> > > input side from the output side currently.
> > > 
> When non-blocking is set, ideally we want to return 0 if there's no hope
> of anymore data and EAGAIN if trying later may yield some data. So how
> about this instead?
> 
> diff --git a/fs/splice.c b/fs/splice.c
> index 9b559ee..0670c91 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -1669,6 +1669,13 @@ static int link_pipe(struct pipe_inode_info *ipipe,
>  		i++;
>  	} while (len);
>  
> +	/*
> +	 * return EAGAIN if we have the potential of some data in the
> +	 * future, otherwise just return 0
> +	 */
> +	if (!ret && ipipe->waiting_writers && (flags & SPLICE_F_NONBLOCK))
> +		ret = -EAGAIN;
> +
>  	inode_double_unlock(ipipe->inode, opipe->inode);
>  
>  	/*
> @@ -1709,11 +1716,8 @@ static long do_tee(struct file *in, struct file *out, size_t len,
>  		ret = link_ipipe_prep(ipipe, flags);
>  		if (!ret) {
>  			ret = link_opipe_prep(opipe, flags);
> -			if (!ret) {
> +			if (!ret)
>  				ret = link_pipe(ipipe, opipe, len, flags);
> -				if (!ret && (flags & SPLICE_F_NONBLOCK))
> -					ret = -EAGAIN;
> -			}
>  		}
>  	}
> 

Thanks! This works great.
Add if you want: Tested-by: Johann Felix Soden <johfel@users.sourceforge.net>

---
Johann Felix Soden

Re: [PATCH] splice: fix problem with sys_tee and SPLICE_F_NONBLOCK

[Jens Axboe]

On Wed, Feb 20 2008, Johann Felix v. Soden-Fr. wrote:
> On Wed, Feb 20, 2008 at 10:35:28AM +0100, Jens Axboe wrote:
> > On Tue, Feb 19 2008, Johann Felix Soden wrote:
> > > 
> > > Am Dienstag, den 19.02.2008, 22:25 +0100 schrieb Jens Axboe:
> > > > On Tue, Feb 19 2008, Johann Felix Soden wrote:
> > > > > > On Tue, Feb 19 2008, Johann Felix Soden wrote:
> > > > > > > From: Johann Felix Soden <johfel@users.sourceforge.net>
> > > > > > > 
> > > > > > > With SPLICE_F_NONBLOCK sys_tee should return number of duplicated bytes,
> > > > > > > not only -EAGAIN on success.
> > > > > > 
> > > > > > ?
> > > > > > 
> > > > > > The current behaviour is to return bytes tee'd, or return -EAGAIN for
> > > > > > zero bytes if SPLICE_F_NONBLOCK is set. It doesn't return "-EAGAIN on
> > > > > > success", not sure what you mean there.
> > > > > > 
> > > > > Sorry, my patch description was not correct. 
> > > > > 
> > > > > The new behavior of sys_tee with my patch is: 
> > > > > 	- return -EAGAIN if there are no data in the pipe, but writer
> > > > > 	  connected to the pipe, 
> > > > > 	- return 0 if there are not writers connected
> > > > > 	- else return number of duplicated byte 
> > > > > 
> > > > > The old behavior was: return -EAGAIN or the number (>0) of duplicated
> > > > > bytes.
> > > > 
> > > > Your patch has an odd way of achieving that goal, modify the real
> > > > location of the assignment instead of overriding something. That has the
> > > > potential to turn into another confusing bug later on, wondering why the
> > > > heck your return value isn't being passed back.
> > > > 
> > > > Improvement is welcome though, you can't distuingish -EAGAIN on the
> > > > input side from the output side currently.
> > > > 
> > When non-blocking is set, ideally we want to return 0 if there's no hope
> > of anymore data and EAGAIN if trying later may yield some data. So how
> > about this instead?
> > 
> > diff --git a/fs/splice.c b/fs/splice.c
> > index 9b559ee..0670c91 100644
> > --- a/fs/splice.c
> > +++ b/fs/splice.c
> > @@ -1669,6 +1669,13 @@ static int link_pipe(struct pipe_inode_info *ipipe,
> >  		i++;
> >  	} while (len);
> >  
> > +	/*
> > +	 * return EAGAIN if we have the potential of some data in the
> > +	 * future, otherwise just return 0
> > +	 */
> > +	if (!ret && ipipe->waiting_writers && (flags & SPLICE_F_NONBLOCK))
> > +		ret = -EAGAIN;
> > +
> >  	inode_double_unlock(ipipe->inode, opipe->inode);
> >  
> >  	/*
> > @@ -1709,11 +1716,8 @@ static long do_tee(struct file *in, struct file *out, size_t len,
> >  		ret = link_ipipe_prep(ipipe, flags);
> >  		if (!ret) {
> >  			ret = link_opipe_prep(opipe, flags);
> > -			if (!ret) {
> > +			if (!ret)
> >  				ret = link_pipe(ipipe, opipe, len, flags);
> > -				if (!ret && (flags & SPLICE_F_NONBLOCK))
> > -					ret = -EAGAIN;
> > -			}
> >  		}
> >  	}
> > 
> 
> Thanks! This works great.
> Add if you want: Tested-by: Johann Felix Soden <johfel@users.sourceforge.net>

Thanks for testing that it works as expected, I'll commit and add your
tested-by.

-- 
Jens Axboe

Re: [PATCH] splice: fix problem with sys_tee and SPLICE_F_NONBLOCK

[Patrick McManus]

On Wed, 2008-02-20 at 10:35 +0100, Jens Axboe wrote:

> When non-blocking is set, ideally we want to return 0 if there's no hope
> of anymore data and EAGAIN if trying later may yield some data. So how
> about this instead?
> 


Thank you Jens and Johann.

-Patrick